Many components in integrated-circuit technology are strongly dependent on process variations and temperature. As a consequence, it is often difficult to create precision analog circuits in standard CMOS technology. Although matching between devices can be highly accurate (greater than 16-bit), typically high absolute accuracy of electrical components can only be achieved by trimming.
However, because the silicon used in integrated-circuit technology is very pure, the thermal properties of chips are often more accurate than their electrical properties. For example, the rate at which heat diffuses through silicon (i.e., the thermal diffusivity of silicon, DSi) is usually essentially process-independent.
One technique for measuring DSi involves an electrothermal filter (ETF). FIG. 1 presents a drawing illustrating an existing ETF, which includes a heater and a (relative) temperature sensor (such as a thermopile), spaced at a distance s, are implemented on the substrate of a silicon chip. During operation of this ETF, heat pulses, which are continuously generated at a fundamental frequency fdrive, diffuse through the silicon and create low-pass-filtered temperature fluctuations across the thermopile. The thermopile converts these temperature fluctuations into a signal VETF with a phase shift φETF (relative to fdrive) given by
      ϕ    ETF    ∝      s    ·                                        f            drive                                D            Si                              .      For s equal to 24 μm, the thermal delay associated with an ETF is about 2.5 μs at room temperature. It has been shown that the device-to-device spread of this thermal delay is mainly a function of the lithographic inaccuracy with which s can be defined. In particular, ETFs with identical s (equal to 24 μm), implemented in 0.7 μm and 0.18 μm CMOS technology, respectively, had untrimmed spreads in their thermal delays of 0.14% (3σ) and 0.045% (3σ). In other words, DSi is insensitive to process spread up to at least this level (greater than 11 bits) and in theory, is expected to be significantly more accurate.
Applications of ETFs include: highly accurate temperature sensors (because DSi is also a well-defined function of absolute temperature T); and frequency references generated by using a loop that locks an oscillator to the thermal delay of an ETF (which are sometimes referred to as electrothermal frequency-locked loops or EFLLs). FIG. 2 presents a block diagram of an existing temperature sensor based on an ETF. When the ETF is driven at a constant frequency, its phase shift φETF is a function of temperature (approximately proportional to T0.9). A phase digitizer converts φETF to a digital representation of temperature. If an accurate clock, such as a clock generated by a crystal oscillator (which are typically available in digital systems), is used to drive the ETF, then the untrimmed temperature-sensing inaccuracy of an ETF-based temperature sensor (over the military temperature range of −55 to 125 C) can be as low as ±0.2 C (3σ). Because ETFs do not suffer from the electrical non-idealities that are typically observed at extremely low or high temperatures, their operating range (and, thus, the operating range of ETF-based temperature sensors) can exceed −70 to 200 C.
FIG. 3 presents an existing frequency reference based on an EFLL. During operation of this frequency reference, feedback forces the oscillator to output a frequency at which φETF equals φSET. However, because φETF is temperature dependent, an integrated temperature sensor is typically required to compensate for the temperature dependence of DSi. For example, the integrated temperature sensor can be based on the well-known temperature dependence of bipolar junction transistors (BJTs). In this way, 0.1% (1000 ppm) 16 MHz frequency references have been fabricated. Nonetheless, the large leakage currents of BJTs at high temperatures gave rise to temperature-sensing errors as large as ±3.0 C at 200 C. As a consequence, the operating range of this frequency reference was limited to −55 to 125 C.
While the performance of the ETF-based temperature sensor and the EFLL-based frequency reference are comparable to the state of the art, problems remain. In the case of the ETF-based temperature sensor, the need for an accurate frequency reference usually precludes stand-alone operation. This constraint often limits ETF-based temperature sensors to applications such as the thermal management of digital chips (e.g., microprocessors), where an accurate frequency reference is readily available. While this is a large and growing market, there is also an increasing need for accurate and robust stand-alone temperature sensors in other applications (such as: automotive, industrial and space applications), which are usually not serviced by existing integrated temperature sensors. However, accurate frequency references are often unavailable in these other applications. While this problem can, in principle, be addressed by including an oscillator on the chip to provide a clock, in practice such oscillators usually have poor absolute accuracy and poor temperature dependence because of component tolerances, which can greatly increase the measurement error (often to much more than ±1 C).
The use of EFLL-based frequency references is often limited by their inaccuracy, which is largely determined by the strong temperature dependence of DSi. This strong temperature dependence imposes strict inaccuracy and noise requirements on the temperature sensors used to temperature-compensate EFLL-based frequency references. Indeed, even if the ETF was perfectly accurate, the use of a state-of-the-art ±0.1 C temperature sensor would still limit the frequency inaccuracy of EFLL-based frequency references to about 800 ppm. This is a problem because most frequency reference applications require inaccuracies between 10 and 100 ppm.
Therefore, there is a need for an ETF-based temperature sensor and an EFLL-based frequency reference without the problems listed above.